That totally depends on the type of insulin you are using. If you have long lasting insulin, it is not meant to be injected at every meal. If you are using fast acting insulin, you need to talk to your doctor and see what they are recommending. Generally you take it 15 minutes before eating, but there is also the scale of how much based on how many carbs you are eating.

when I was on MDI I injected every time I ate, plus Lantus at night, plus any corrections. Do you have a good endo that can help you figure your IC ratio and correction factor? You need to know how your body reacts to carbs and insulin, not everyone needs to inject for snacks, especially depending on what the snack is, but some have to inject for everything. Test your blood a lot, write everything down, this will help you find patterns.